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Inorg. Chem. 1982, 21, 4100-4101
dependent pathways remain the more rapid step due to the large thermodynamic driving force of the redox reaction derived from the couple HMn04/HMn04- (EO = 1.1l V).2b Although the form of the rate law provides no detailed information concerning an inner- or outer-sphere mechanism, the aquomolybdenum(V) dimer has been compared in recent studies to aquovanadium(1V). lg20 Both ions are d' systems and are similar in their overall structure as both contain terminal oxo groups. Solid-state structural data show a lengthening of the bond to the trans ligand for various compounds of both ions, and it is thought that the terminal oxo group weakens the bond trans to it.2' This weakened bond is felt to be responsible for the rapid exchange of the trans ligand with solvent in Mo204(NCS)64-.22A comparison of the rate constants from the expression k2 = ko k',K,,[H+] for the reaction of Mn04- with the aquo dimer and V02+ shows the dimer is more rapidly oxidized. The sizes of the ratios ko(dimer)/ ko(VOz+) = 74 and kh(dimer)/ k(,' V02+) = 10 appear to suggest a similar mechanism for these ions with Mn04-.23324 Sykes has called the (H20)6M02042+ion an outer-sphere reductant from comparison of rate and thermodynamic data from the reaction of M%O?+ and M O ~ O ~ ( E D T Awith ) ~ - those for the reaction of IrC162-and F e ( ~ h e n ) ~both ~ + , outer-sphere oxidants.I3 Table IV presents data from the oxidation of Mo20?+ and V02+by Mn04- and several other oxidants. It is interesting to note that, while the values of AH*for the reactions of Mn04- with M o ~ O and ~ ~ V02+ + are similar and not unlike those of the M ~ ~ O ~ ~ + / F e ( p h reaction, e n ) ~ ~ +the activation entropy, M*, values differ considerably. It has been s h o ~ nthat ~ ~ AS* , ~ values for a group of outer-sphere reactant/Mn04- reactions tend to be much more positive (-5.6 to +36.5 cal/(mol K)) than AS*for a set of inner-sphere reactants (-13 to -30 cal/(mol K)).26 An isokinetic plot using AH*and M*values from various reductant/Mn04- reactions also shows the aquo dimer behaves as an outer-sphere reductant and not an inner-sphere reactant like V02+. Since the aquo Mo(V) dimer can be considered a oneelectron reductant with Mn04-, an Eo value was estimated for the reaction MoV2 MoVMoV* e-, from a plot of the rate constants for the unprotonated pathways, log ko, vs. Eo(reductant) for a series of Mn04-/inorganic ion reactions.27 The value, -0.53 V, is in reasonable agreement with the one-electron value of -0.52 to -0.56 V estimated by Pope from studies of Mo(V)-substituted "Keggin anions"." Results (AH*, kcal/mol; AS', cal/(mol K)) from complexation studies of SCN- with ( H 2 0 ) 6 M ~ V(11.3, 2 -0.3) and V 0 2 + (10.8, -3.7) have been interpreted as indicating substantial involvement of the equatorial position with the incoming ligand.lg It is likely that the interaction of the permanganate ion with these two ions is similar. We are presently examining this idea.
+
-
+
(18) N . Bailey, A. Carrington, K. A. K. Lott, and M. C. R. Symons, J . Chem. Soc., 290 (1960). (1 9) Y.Sasaki, R. S. Taylor, and A. G . Sykes, J. Chem. Soc., Dalton Trans., 396 (1975). (20) R. K. Murmann, Inorg. Chim. Acta, 25, L43 (1977). (21) D. L. Kepert, "The Early Transition Metals", Academic Press, London, 1972, p 318, and references within; K. B. Swedo and J. H. Enemark, J. Chem. Educ., 56, 70 (1979). (22) S. Wajda and A. Zarzeczny, Nukleonika, 19, 33 (1974); Chem. Abstr., 81, 69217r (1974). (23) J. P. Birk and S. V. Weaver, Inorg. Chem., 11, 95 (1972). (24) J. P. Birk, Inorg. Chem., 16, 1381 (1977). (25) D. R. Rosseinsky and M. J. Nicol, J . Chem. Soc. A, 1022 (1968). (26) F. Freeman et al., J . Am. Chem. Soc., 103, 1154 (1981). (27) W. L. Waltz, University of Sasketchewan, unpublished results on the oxidation on a series of cyanometal complexes by the OH radical.
0020-1669/82/1321-4100%01.25 IO
Acknowledgment. This research was supported in part by funds from an Eastern Michigan University Summer Research Fellowship (K.W.H.), and National Science Foundation Summer URP Grant Nos. 20320 and 20321. Registry No. MnO,-, 14333-1 3-2; M o * O ~ ~54429-28-6. +,
Contribution from the Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439
Molecular Orbital Calculation of Migration Barriers in tbe LiAIF4 and MgAIFSComplexes' L. A. Curtiss Received February 8, 1982
Numerous metal halide complexes of the type MAlX, (M is a mono- or divalent metal; X is a halogen) have recently been studied in the vapor phase by various physicochemical techniques.2 Experimental evidence has indicated that these complexes exist as edge-bridged structures in which the two metal atoms are bridged by two of the halogen atoms. There has also been some experimental work suggesting that these complexes have face-bridged structures (three halogens in the bridge) or C, structures (a nonplanar edge-bridged ring). Questions as to the exact structure of these MAlX, complexes and the mobility of the cation fragment (MX,,') in the complex, which could make structural determination difficult, have not yet been completely resolved. In previous work we addressed the question of the relative stabilities of corner- (one fluorine in the bridge), edge-, and face-bridged structures of MAlX,-type complexes by carrying out ab initio molecular orbital calculations on LiA1F4,3BeAlF5,4 and MgA1F5.4 In all three cases the edge-bridged structure was most stable (by 5-25 kcal mol-') and could be described as an AlF4- anion interacting with an MF-' cation (LP, BeF+, or MgF+). Others5 have similarly found the edge-bridged structure for the related LiBeF3 complex to be most stable. In the work presented here we have calculated energies of points on the pathway for migration of the cation between edge, face, and corner bridges in LiAlF4 and MgAlF5. The motivation for these calculations was to shed light on the following unresolved questions concerning MAlX, complexes: (1) Are the less stable bridging structures (face and corner bridges) metastable enough to be observed; i.e., what are the barriers for transition from the less stable structures to the equilibrium structure? (2) Does the potential energy of this path allow for easy migration of the cation over the entire anion MX4-? (3) Is the potential energy curve in the region of the edge-bridged structure shallow enough to explain the C, structure observed in electron diffraction studies? Computational Methods Calculations were carried out by using standard LCAO-SCF methods6 with an extended-basis set. The 6-31G basis was used on (1) Work was performed under the auspices of the Material Sciences Office of the Division of Basic Energy Science of the Department of Energy. (2) M. Hargittai and I. Hargittai, "The Molecular Geometries of Cwrdination Compounds in the Vapour Phase", Elsevier, Amsterdam, 1977. (3) L. A. Curtiss, Chem. Phys. Left., 68, 225 (1979). (4) L. A. Curtiss and A. Heinricher, Chem. Phys. Lett., 86, 467 (1982). (5) (a) V. G. Zakzhevskii,A. I. Boldyrev, and 0.P. Charkin, Chem. Phys. Left., 70, 147 (1980). (b) E. U. Wurthwein, M. B. Krogh-Jespersen, and P. v. R. Schleyer, Inorg. Chem., 20, 3663 (1981). (6) All calculations were performed by using the GAUSSIAN 76 series of programs: J. S. Binkley, R. A. Whiteside, P. C. Hariharan, R. Seeger, J. A. Pople, W. J. Hehre, and M. D. Newton, Program No. 368, Quantum Chemistry Program Exchange, Indiana University.
0 1982 American Chemical Societv
Znorg. Chem. 1982, 21, 4101-4103
I 2o
t
Corner-bridged
I11
and corner-bridged structures may be closer in energy to the edge-bridged structure (e.g., 20°) over the AlF; anion at temperatures up to 2000 K. It should be noted that the height of the potential energy curve could decrease with a more sophisticated calculation (i.e., a bigger basis set or inclusion of correlation energy). Also, in some MAK, systems the face
--
(7)
4101
-
W.J. Hehre, R.Ditchfield, and J. A. Pople, J. Chem. Phys., 56, 2257
(1972). (8) J. D. Dill and J. A. Pople, J. Chem. Phys., 62, 2921 (1975). (9) T. H. Dunning, Jr., and P. J. Hay, Mod. Theor. Chem., 3, 1 (1977). (10) J. L. Gole, A. K.Siu, and E. F. Hayes, J. Chem. Phys., 58,857 (1973).
-
~~
~~~~~~
(1 1) In the case of LiAIF, optimization of the estimated geometry reduced the energy by up to 2 kcal mol-' for points between I1 and 111.
Contribution from the Departments of Chemistry and Biochemistry, University of California, Los Angeles, California 90024
The Crown Ether Promoted Base Degradation of p -Carborane David C. Busby and M. Frederick Hawthorne* Received April 21, 1982
In 1964 Wiesboeck and Hawthorne' discovered that degradation of the closo icosahedral carborane 1,2-C2B1a12by ethanolic KOH produces a novel nido carborane anion, [7,8-C2BgH12]-,whose structure is that of an icosahedron with one vertex removed (Figure 1). Similar treatment of 1,7C2B,,,HI2 leads more slowly to the isomeric [7,9-C2BgH12]-. In both cases, the boron atom removed is one that is adjacent to both electronegative carbon atoms, which makes the boron more susceptible to nucleophilic attack. Strong base removes the bridging proton from the open face of [7,8-C2B9H12]-and [7,9-C2BgH12]-to give nido dianions that can bond to metals in an q5 fashion.2 This discovery led to the development of the field of metallacarborane chemistry. 'To whom correspondence should be addressed at the Department of Chemistry.
0020-1669/82/ 1321-4101$01.25/0 0 1982 American Chemical Society
